Weathering: Definition and Types- Physical, Biological and Chemical Weathering

There are two types of the processes that affect the landforms viz. Exogenic and Endogenic.

• Endogenic are the processes that occur within the earth’s surface such as Plate tectonics, earthquakes, volcanoes etc.
• Exogenic are the processes that occur on or near the earth’s surface. The tidal force is Exogenic. The radiation from Sun is also Exogenic.

Further, there are 3 Exogenic geological processes which refer to the process of disaggregation which lead to the reduction in the elevation & relief of the landforms and landscapes such as rocks and mountains. These 3 important phenomena are weathering, mass wasting and erosion. These all together are called “Degradation” or “Denudation“.  Endogenic processes uplift and expose continental crust to the Exogenic denudation. Exogenic denudation works in opposition and reduces landscapes to sea level.

Weathering is the breaking down of Rocks, soils and minerals through “direct Contact” with the atmosphere of the earth. It occurs in situ, means there is no movement involved.

Erosion

This is distinct from erosion which involves the movement of rocks and minerals such as water, ice, wind and gravity.

Mass wasting

Mass wasting involves the movement of the rocks and particles across a slope due to gravity.

Weathering

Weathering refers to the combined action of all processes that cause rock to disintegrate physically and decompose chemically because of exposure near the Earth’s surface. Weathering produces regolith. Weathering also creates a number of distinctive landforms.

• Regolith is a surface layer of weathered rock particles that lies above solid, unaltered rock.

Weathering is the in situ disintegration and breakdown of rocks, soils and minerals.

Types of Weathering

There are three types of weathering viz. Mechanical or Physical Weathering, Chemical Weathering & Biological Weathering.

Physical Weathering

Physical Weathering can be caused by thermal changes, Frost Action, Pressure Release, Hydraulic action and Haloclasty. These terms have been discussed below:

Thermal Changes

Repeated changes in the temperature (heating and cooling) exert the stress on the outer layers of the rocks which is called as Thermal Stress. The rocks expand when there is a rise in the temperature and contract when there is a fall in the temperature. In deserts, the phenomena are more common as there is large diurnal temperature range. The Forest fires can raise the temperature suddenly and this leads to thermal shock.

Thermal Expansion versus Thermal Contraction

Please note that all materials respond by changing volumes because of temperature. Most materials expand when there is a rise in temperature. But there are some rare example which contract when temperature increases and expand when temperature decreases.  This is called Thermal contraction.

The coefficient of thermal expansion is positive for the material which expand when there is a rise in temperature. If the coefficient of thermal expansion becomes zero, there is no expansion or contraction. At negative coefficient, the material contracts when there is a rise in temperature. Best example is water. Water when cooled till 4°C, the coefficient of thermal expansion decreases and become zero at 4°C. After that, when temperature is further reduced, it expands. So, at 4°C, water has maximum density. After that, density is reduced and this is the reason why ice floats and the water bodies are able to retain a temperature of 4°C at sub zero weathers. Similarly, Pure Silicon has a negative coefficient of thermal expansion between -255°C to -153°C.

Frost Action

One of the most important physical weathering processes in cold climates is frost action. As water in the pore spaces of rocks freezes and thaws repeatedly, expansion can break even extremely hard rocks into smaller fragments. Water penetrates fractures in bedrock. These fractures, called joints, are created when rocks are exposed to heat and pressure, then cool and contract. Joints typically occur in parallel and intersecting planes, creating natural surfaces of weakness in the rock. Frost action then causes joint-block separation. Water invades sedimentary rocks along their stratification planes, or bedding planes.

Pressure Release or exfoliation

This refers to the release of the pressure from unloading of existing rock on the rocks that lie beneath it due to other processes such as erosion. The igneous rocks are formed deep in earth and when the rocks above them get removed, the igneous rocks expose and the pressure is released. This causes their outermost surfaces to expand. This expansion leads to weathering.

Hydraulic Action

This phenomenon takes place due to very high powered water waves. When water rushes into cracks in the rocks with a very fast speed, the trap of air in the cracks get compressed and thus weakens the rocks. When water retreats, the trapped air is suddenly released with explosive force.

Salt Crystallization or Haloclasty

This refers to the process in which the rocks are denudated due to salt formation. This is a two step process. The first step is started when saline water seeps into cracks and evaporates depositing salt crystals. In the second step, when the rocks are heated up, the crystals expand putting pressure on the surrounding rock. Over the period of time, it splinters the stone into fragments.

Biological Weathering

Biological Weathering refers to the contribution made by the organisms such as Lichens and mosses, which grow on essentially bare rock surfaces and create a more humid chemical microenvironment. Biological weathering is both physical as well as chemical breakdown of the surface micro layer of the rock. The animals such as earthworms and other annelids, moles, rabbits all contribute to the biological weathering.

Chemical Weathering

Chemical weathering refers to the changes in the chemical composition of the rocks and generally refers to the chemical reactions of water with minerals.

Hydration

Hydration means absorption of water by some kinds of rock, leading to expansions and disintegrations. When water molecules bind with the mineral molecules, it is called Mineral Hydration.

Hydrolysis

The chemical breakdown of the rocks caused by rainwater is called Hydrolysis. The result may be secondary minerals with different chemical structure.

Oxidation

Oxidation or rusting occurs when atmospheric oxygen reacts with the minerals such as Iron Ores. This leads to decomposition of the rocks.

Solutions

This refers to dissolving of the minerals in water.

Carbonation

Carbonation refers to the chemical weathering in which Carbon dioxide attacks the rocks after it makes weak acid reacting the water.  The rocks are generally made up of calcium carbonate such as Limestone and Chalk.